Forums are online communities. Most forum sites are highly vertical and focused on a topic. There are a couple of things to keep in mind when starting a forum. First, it takes consistent effort to get them off the ground. Second, they're highly transient communities for the majority of users with a staple of a few passionate and frequent users.

How to drive traffic to a forum

1. Start with knowledgable people. People that know the subject matter very well and have them seed the forum.
2. Be patient. Communities grow over time. Just, consistently post new content.
3. Link development. People need to know that your forum exists. Post links in your blog, and other appropriate places.
4. Build more content.
5. Submit to a few key directories.
6. Recruit people to post.
7. Post on a topic and then email people you know that will have commentary.
8. Maintain the forums
9. Reward people for posting